Method of making electrical paper



United States Patent 3,143,458 METHOD 0F MAKING ELECTRICAL PAPER Charles Andre Chan-on, Westfield, Mass, assignor to Th Stevens Paper Mills, Inc, Westfield, Mass., 2 corporation of Delaware No Drawing. Filed Oct. 24, 1961, Ser. No. 147,187 1 Claim. (Cl. 162-76) This invention relates to electrical paper of the type used as the dielectric material in the manufacture of capacitors and processes for making the same.

The principal object of this invention is to provide electrical paper having improved dielectrical properties such that capacitors in which the paper is used have superior power factors.

Another object of this invention is to provide more eficient and economical processes for manufacturing electrical paper.

As is well known, the power factor of an inductor, capacitor or insulator is an expression of its electrical losses. The power factor of capacitors in which paper is the dielectric material is affected by the presence of ions in the paper. Conventionally, during the manufacture of electrical paper, washing is employed as the means for removing ions. Washing is carried out throughout the paper-making process using water specially de-ionized for the purpose.

Notwithstanding the use of de-ionized water throughout the paper-making process, metallic ions are present in sufficient quantities to exercise an adverse effect on the dielectric properties of the paper which manifests itself in a relatively high power factor and short life of the capacitors.

In accordance with this invention, processes have been developed for producing electrical paper having an ion content substantially lower than heretofore available. The processes include the use of a chemical which reacts with the ions in such a way as to chemically and electrically inactivate the ions. Chelating agents have the requisite characteristics for this purpose and are characterized by a molecular structure in which a ring compound can be formed by residual valencies of available atoms. Chelating agents will react with metallic ions to form compounds having more than one bond between the metallic atom and the molecule of the chelating agent. After the metallic ions have reacted with the chelating agent, the ions lose their chemical and ionic identity and are no longer chemically or electrically active. Thus their presence as part of a ring compound will not afiect the dielectric characteristics of the paper. A chelating agent may be added at any time during the paper-making process or during or after refining.

One example of a chelating agent which may be used in carrying out this invention is ethylene-diamine-tetraacetic acid, hereinafter referred to as EDTA, represented by the following structure:

When a chelating agent such as shown above encounters metallic ions such as Al+++, Ca Cu++ and the like,

"ice

a ring compound is formed such as represented by the following chemical structure:

0 0 II 5 HO-O-CH onFoH, om-d on In a ring compound such as this, metallic ions, illustrated as copper in the above example, are electrically and chemically deactivated.

This invention may be carried out by continuous processing, by batch processing, or a combination of continuous and batch processing. In continuous processing the paper slurry being fed to the paper-making machinery is continuously treated with an amount of chelating agent equivalent to .00l%1% based on the dry weight of the pulp. It has been found that the use of chelating agents in amounts greater than 1% as compared to the total dry paper stock does not result in corresponding improvement in the dielectric properties of the paper and may cause difi'iculties in removal. The chemical may be introduced into the slurry either in a dry powder form or dispersed in suitable liquid media, such as Water, and may be introduced at room temperature since temperature is not a critical factor in the use of chelating agents. To

obtain maximum effect throughout the processing of the pulp, the chemical may be introduced in a number of different locations in the process. 7

In accordance with the batch processing method, the pulp, in slurry form, is agitated for periods of from 10-60 minutes at a consistency of from 15%. A chelating agent is added to the slurry in an amount from .00110% relative to the total weight of the slurry. Thereafter, the

pulp slurry is thoroughly washed to remove chelated ions or ring compounds; this may be carried out in conventional equipment. Washing is an important step in the process since it is desirable to remove as much of the ring compounds as practicable because it has been found that such compounds have an adverse effect on the power factor of paper. The presence of EDTA in the paper is indicated by the extract resistivity of the paper. In accordance with this invention washing is continued until the extract resistivity of the paper is not less than 600,000

and preferably greater than 900,000 ohms/cm.

Ring compounds formed by reaction of metallic ions and chelating agents are quite soluble in Water and therefore readily removed from the paper stock by washing. However, the unreacted EDTA is not so soluble and not as easily removed. Because of the effect of unreacted EDTA on power factor it is preferable that only enough EDTA be used to react with ions present in the pulp slurry. This will minimize the amount of unreacted EDTA to be removed.

Electrical papers of various densities, having substantially improved dielectric characteristics, have been at- 60 tained by the addition of .188% EDTA by weight of the total weight of the dry paper stock. The papers produced contained 7 to 97% alpha cellulose, 2.76% pentosans, ash of .05.25%, and an extract resistivity of not less than 600,000 ohms/cm. Tests also showed that the 1 density paper had a cation content not greater than 800 parts per million (p.p.m.) and .7 density paper had a cation content not greater than 373 ppm.

Tests of paper made in accordance with this invention show a substantial reduction in the mineral content and substantially improved power factors as compared with identical types of paper which are made using conventional washing processes. The table below shows a comparison of the ash content and the power factor of various electrical papers classified as to density. Under the headings Percent Ash and Percent Power Factor,.the left hand column represents conventional Wash.- ing processes and the right hand column represents a paper made in accordance with this invention. The difference between the power factors is shown in the right hand column. This power factor improvement is a function of the chelation of cations in the paper pulp and may be called the chelation factor of the paper.

Percent Ash Percent Power Percent Factor Power Fac- Type of Paper (density) tor Loss (Chelation Wash EDTA Wash EDTA Factor) Process Process Process Process The power factor tests listed above were carried out using a Wien bridge tester operated at 366 volts per mil thickness. of paper, 60 cycle, and at a temperature of 125 C.

Further analysis of electrical papers manufactured in accordance with this process was conducted to determine the effects of chelating agents on the various types of metallic ions. The results of the test are listed below:

The above test shows a substantial overall reduction from 793 to 505 in the'quantity of the metallic content of paper manufactured using a chelating agent as contrasted with the conventional washing technique using deionized water.

In the manufacture of ultra purity electrical papers it is usual practice to employ high quality pulps which have been thoroughly washed. There are available on the market a number of less expensive pulps, similar in quality to the aforementioned pulps, with the exception of cleanliness. It has been found that by using EDTA, the less expensive pulp can be cleaned successfully so that it can be used in making ultra purity capacitor paper. Moreover, improvement of the power factors of such papers of as much as 25% has been noted.

Another important advantage of this invention is the improved life characteristics of capacitor paper. The life of such paper may be determined by a number of different breakdown tests conducted under stresses substantially higher than the final capacitor application requires. One of the many factors which affects the life of a capacitor is its ionic content. At high temperatures electro-chemical action contributes to paper failure and capacitor breakdown. Thus a paper which is free of ionic contamination will have a longer life and higher reliability than similar paper having greater ion content.

A .0004 normal density paper made in accordance with this invention was wound into capacitors and compared in performance with untreated paper. The capacitors were tested by means of a standard test used in the paper making industry in which high voltage stresses and high temperatures are employed to accelerate capacitor breakdown. It was found that 20% of capacitors made with the untreated paper failed in 33 hours testing, and failed after 54.5 hours testing. In contrast the treated paper showed 20% failure after 68 hours of testing, and 70% failure after 107.4 hours. These figures show that the breakdown life of EDTA treated paper was increased approximately 200%.

During the testing of EDTA it was found that the chelating agent caused a remarkable increase in the water drainage rate from stock slurries. While it is well known that certain chemicals may be used to increase the drainage rate of water from stock slurries, these chemicals frequently cause deterioration or contamination of the paper. In the manufacture of capacitor paper, refining treatments are required which result in slow draining stock slurries. This has always been considered a limiting factor on the speed of the paper-making machine on which this type of paper is produced. By using EDTA, however, a marked increase in the drainage rate was noted even for normally slow draining pulps. The drainage rate increase was so great in fact, that the speed of the paper-making machine could be substantially increased. The results were that low density grades of electrical paper were manufactured at 10% higher speeds and high density grades of paper were produced at 15% higher speeds than heretofore considered possible.

Having thus described my invention what is claimed is:

In the manufacture of electrical paper for use as dielectric material in capacitors the method of improving the power factor of said paper which comprises the steps of introducing into the pulp slurry ethylene-diamine-tetraacetic acid to remove cations therefrom, the quantity of said ethylene-diarnine tetraacetic acid being sufiicient to react with said cations to form ring compounds, thereafter washing said pulp to remove said ring compounds, testing the extract resistivity of the paper, and continuing said washing until said resistivity is at least 600,000 ohms/cm.

References Cited in the file of this patent UNITED STATES PATENTS 2,505,545 Hopkins Apr. 25, 1950 2,707,145 Sparrow Apr. 2 6, 1955 2,934,686 Sproull Apr. 16, 1960 OTHER REFERENCES Sequestrene, pub. by Geigy Industrial Chemicals, Ardsley, N.Y., 2, pp. 1 and 30.

Tappi, Series 27, 1944, pp. 438-440, December 28, 1944 (TS1080T3). 

